This invention relates to a method for the dehalogenation of halogenated aliphatic and alicyclic hydrocarbons in aqueous compositions. In one aspect, this invention relates to a method utilizing a mixed metal composition to dehalogenate halogenated aliphatic and alicyclic hydrocarbons in an aqueous composition. In another aspect, this invention relates to a method of contacting a mixed metal composition prepared by contacting a metal or a soluble metal salt with an anchor metal to deposit the metal on an anchor metal with a halogenated aliphatic and alicyclic hydrocarbon contaminated aqueous composition to dehalogenate the halogenated aliphatic and alicyclic hydrocarbon. In a further aspect, this invention is utilized in situ to dehalogenate halogenated aliphatic and alicyclic hydrocarbons in groundwater.
Halogenated aliphatic and alicyclic hydrocarbons, particularly chlorinated aliphatic and alicyclic hydrocarbons such as carbon tetrachloride, chloroform, trichloroethylene and tetrachloroethylene, have been widely used as chemical intermediates, solvents for dry cleaning of clothing, degreasing, and a variety of other applications. Halogenated aliphatic and alicyclic hydrocarbons are very stable and persistent in the environment, and are relatively toxic at low levels.
Over the years, halogenated aliphatic and alicyclic hydrocarbons have been accumulating in the environment, particularly in groundwater. As a result, pollution of water by chlorinated hydrocarbons has become an important environmental problem and contaminated groundwater represent a large portion of environmental action plans throughout the world.
It is known that chlorinated compounds can be degraded by reductive dechlorination, i.e. replacement of chlorine substituents by hydrogen. Evidence indicates that many of the chlorinated solvents are degraded under anaerobic conditions in sediments and in systems simulating anaerobic wastewater treatment. Current theory suggests that some of these anaerobic dechlorinations are not strictly biological in nature, but are catalyzed by metallic iron. While the use of iron in the dechlorination affords little environmental threat, its use suffers from certain problems.
Recently, Gillham and O'Hannesin disclosed in U.S. Pat. No. 5,266,213, a method for cleaning halogenated contaminants from groundwater. The process involves feeding contaminated groundwater through a trench containing a metal such as iron, under strict exclusion of oxygen, and over a lengthy period of time. Disadvantages of this system include the fact that large amounts of iron and substantial periods of time are needed for completion of the reactions.
In U.S. Pat. Nos. 4,219,419 and 4,382,865 (Sweeny), there is disclosed a system for treating the effluent created during the production of halogenated pesticides. In that process, the effluent water stream containing the halogenated waste material is required to be buffered to near neutral pH prior to passing the waste stream over a metal couple.
There is a need for an enhanced method for dehalogenating aliphatic and alicyclic hydrocarbons in contaminated aqueous solutions so as to diminish the amount of metallic iron needed to complete the dehalogenation. There is also a need for a dehalogenation method where the rate of dehalogenation is increased and innocuous reaction products are generated. It has now been found that a mixed metal composition prepared by contacting a metal or a soluble metal salt with an anchor metal to deposit the metal on an anchor metal while controlling the portion of the surface area of the anchor metal which has a second metal deposited thereon solves the above-described problems.